The invention relates to a tumble dryer comprising a drying chamber for the articles to be dried, a process-air duct, in which is located a heater for heating the process air and in which the heated process air can be directed by a first fan over the articles to be dried in the drying chamber, and comprising a heat pump having a heat sink, a heat source and a heat transfer device, and also a controller, and relates to a preferred method for operating said tumble dryer.
A conventional tumble dryer, whether or not it comprises a heat pump as a means of partial recovery of the thermal energy used to dry the washing that is provided, is designed either as a vented dryer or a condenser dryer.
In the vented dryer, ambient air is drawn in from around the tumble drier, heated and conducted once over the washing to be dried for the purpose of absorbing moisture, and then ducted out of the tumble dryer. Since it is moisture-laden, the air cannot simply be vented into a building in which the tumble dryer is installed, but must be ducted out of the building in a controlled manner via a venting hosepipe or the like. A tumble dryer in the form of a condenser dryer carries the process air used for drying the washing in a closed circuit, where it follows a cycle in which it is heated and directed over the washing and then cooled in order to condense the moisture that it is carrying and to separate it from the process air as condensate; the process air is then heated and guided over the washing again. A condenser dryer does not need a venting hosepipe and is very popular for installing in a bathroom with no outside walls or a laundry room with no outside walls and also in large apartment blocks.
In the condenser dryer (also referred to below as a “dryer” for short), the process air is conducted by a fan over a heating device into a rotating drum as the drying chamber containing damp laundry. The hot air takes the moisture out of the laundry to be dried. After passing through the drum, the now moist process air is conducted into a heat exchanger or another heat sink, in front of which is usually connected a lint filter for catching a lint, i.e. fine, suspended fabric particles that the process air draws from the washing to be dried. In the heat exchanger (e.g. air-air heat exchanger) or the heat sink, the moist process air is cooled so that the water contained in the moist process air condenses. The condensed water is then usually collected in a suitable container. The cooled and dried air is fed again to the heating device and then to the drum.
Each of the drying processes just described uses a large amount of energy, in the case of the vented dryer to take away the heated and moisture-laden airflow from the vented dryer after a single pass through the moist laundry, or in the case of the condenser dryer, in which a heat exchanger is used that is cooled by a cooling airflow or the like, because the heat removed in cooling the process air in the heat exchanger is conducted away in the cooling airflow and hence is lost to the drying process. This energy loss can be reduced significantly by using a heat pump instead of the heat exchanger. In a condenser dryer equipped with a heat pump, the warm, moisture-laden process air is cooled largely in a heat sink of the heat pump, for instance in an evaporator for a working fluid carried in a circuit in a heat transfer device, where the transferred heat is used to evaporate the working fluid. The heat-pump refrigerant vaporized because of the heating action is fed via a compressor in the heat transfer device to a condenser for the working fluid, which acts as a heat source, where the condensing of the gaseous working fluid releases heat that is used to heat the process air before entering the drum. The working-fluid circuit is closed in this heat pump by the working fluid returning to the evaporator via an expansion valve in the heat transfer device, where it expands to a lower internal pressure. Other forms of heat pump are known.
DE 40 23 000 C2 discloses a tumble dryer containing a heat pump of the type described above, in which an air-intake aperture is arranged in the process-air duct between the condenser and the evaporator, where said aperture can be closed by a controllable closure device.
DE 197 38 735 C2 describes a condenser dryer having a closed drying-air circuit, which is likewise equipped with a heat pump. The heat pump is designed on the absorber principle and its absorber forms a third heat exchanger, with the refrigerant flowing through its primary circuit, and the drying air flowing away from the second heat exchanger being fed via its secondary circuit back to the secondary circuit of the first heat exchanger.
Many known tumble-dryer heat pumps use compressors for cyclically vaporized and condensed working fluids. These fluids usually work best in specific temperature ranges. The problem with using such a compressor in the condenser dryer is that there are often high temperatures in the condenser, which by the nature of the process mean that the compressor needs to be switched off and/or the heat-pump efficiency worsens. This problem is even greater if an additional conventional heater, in particular an electric resistance heater, assists the compressor in the process-air circuit in order to achieve faster heating of the process air and/or shorter drying times. Hence it is often necessary to reduce the temperatures in the heat pump, for example by using an additional fan to produce a cooling airflow.
The commonly used air-air heat exchanger, whether operated in a cross-flow or counter-flow arrangement, and the electric heater are generally replaced together by a heat pump. This can achieve efficiency improvements of 20% to 50%. Extremely energy-efficient drying is possible in a dryer of this type. In general however, it cannot be used to achieve rapid drying. Moreover, although known conventional driers can usually dry damp laundry very quickly, this means that they also often have a very high energy consumption.
There are also driers in which a heat pump is used that contains a small compressor or a small refrigeration circuit, where a lack of heating or cooling power is made up for by an electric resistance heater or an air-air heat exchanger. A dryer of this type can be operated solely using the heat pump, using the heat pump and the electric resistance heater or using the resistance heater and/or the air-air heat exchanger.